Age hardenable stainless steel alloy



% NICKEL OR NICKEL. EQUIVALENT 1968 L. P. MYERS ET Al. 3,498,178

AGE HARDENABLE STAINLESS STEEL ALLOY Filed June 27, 1967 I i l l l l s 910 I: l2 l3 United States Patent 3,408,178 AGE HARDENABLE STAINLESSSTEEL ALLOY Lewis P. Myers, Mount Penn, and Kermit J. Goda, Jr.,

Leesport, Pa., assignors to The Carpenter Steel Company, Reading, Pa., acorporation of New Jersey Continuation-impart of application Ser. No.401,831,

Oct. 6, 1964. This application June 27, 1967, Ser.

10 Claims. (Cl. 75-425) ABSTRACT OF THE DISCLOSURE A high strengthstainless steel alloy age hardenable to a hardness of at least 45Rockwell C with no more than about 10% retained austenite containing nomore than 0.2% carbon, 8-14% chromium, -12% nickel, up to 7% copper, 0.6to 2% titanium, no more than 0.5% aluminum, 0.25 to 1% columbium, up to0.1% boron, and the remainder iron in which the chromium and nickel ornickel equivalent content of the alloy are balanced as indicated by thearea ABCDEF in the drawing, in which the nickel equivalent content iscalculated by adding onehalf the percent copper to the percent nickel,and in which the copper may replace no more than about 4% nickel.

This application is a continuation-in-part of our application Ser. No.401,831 filed Oct. 6, 1964, allowed on Mar. 31, 1967, and now abandoned.Except for the larger amounts of carbon that may be present in ouralloy, our present application is identical in all essential respects tosaid application Ser. No. 401,831, now abandoned.

This invention relates to iron-base alloys and more particularly to highstrength stainless steel alloys having exceptionally high notchductility and tensile strength.

Heretofore, stainless steel alloys having good corrosion resistance haveleft much to be desired when high strength was required. The use ofrelatively large proportions of such costly alloying elements as nickeland cobalt has met with some success but the highcost of the alloys thusprovided has substantially limited their usefulness.

We have discovered that by carefully controlling the relativeproportions of the main alloying elements, chromium and nickel orchromium, nickel and copper with respect to each other and together withadditive elements as will be more fully described hereinafter, weprovide an alloy from which parts may be readily fabricated and agehardened to a substantially fully martensitic condition having anoutstanding combination of mechanical and stainless properties. Toachieve these properties as well as good ductility and notch tensilestrength in our stainless steel alloy, we carefully control the elementspresent in our alloy to ensure that in its fully heat treated conditionthere is no more than about and preferably no more than about 1%retained austenite in the alloy.

Important advantages of the present invention reside in the uniquecombination of mechanical and stainless properties in productsmanufactured from our alloy. In addition to high strength, ductility andhigh notch tensile strength, such products are uniformly easily formedand machined while in the solution treated condition prior to aging.When fabricated and heat treated, parts formed from our alloy may beused where good corrosion and oxidation resistance are required. Withcontrolled additions of copper, products made from our alloyare-especially well suited for use where, in addition to high strength,good corrosion resistance to salt spray and atmospheric conditions arerequired. In addition to being characterized by an outstandingly uniquecombination of "ice strength and resistance to stress corrosion crackingin certain media, a further advantage resides in the fact that partsformed of our alloy may be readily welded and are not sensitive toweld-cracking. A high proportion of these properties is provided bysuitably proportioning the elements chromium, nickel, copper, titaniumand columbium, in an iron base alloy in which carbon is kept below a lowlevel and subjecting the alloy to a heat treatment which brings out thedesired strength, toughness, ductility and hardness of the alloy whilepreserving its stainless properties.

In its broader aspects, the present invention provides an alloyconsisting essentially of a maximum of about .2% carbon, about 8 to 14%chromium, about 5 to 12% nickel, about .6 to 2% titanium or titaniumplus aluminum but not more than about .5 aluminum, about .25 to 1%columbium, about 0 to 7% copper, up to about .l% boron, and the balancebeing essentially iron except for small amounts of other elements whichdo not adversely alfect the desired properties of the alloy. Phosphorusand sulfur are not to exceed .05 While manganese up to about 2% andsilicon up to about 1% may be present when the alloy is air melted, theyare usually present only as an impurity when the alloy is melted orremelted under a controlled atmosphere. Here and throughout thisapplication, percent concentrations refer to percent by weight.

Within the ranges stated, it is essential, in order to achieve amongother things the high strength and ductility of our alloy, that theamounts of chromium, nickel and copper present in our alloy be carefullybalanced. Unless the combined total of chromium and nickel or chromiumand nickel plus one-half the copper content of our alloy is at leastequal to 18%, the high notch tensile strength and ductilitycharacteristic of our alloy is not attainable. In our alloy, copper,which may be present in amounts ranging up to about 7% and may be usedto replace up to about 4% of the nickel, is only about one-half aseffective as nickel in its effect on causing the formation of austenitein the alloy. Thus, 1% of copper is equivalent to .5 of nickel in thisrespect and in computing the nickel equivalent content of our alloy,one-half of the copper content is added to the nickel content. As thechromium conent of our alloy is raised from about 10% to about 14%, themaximum amount of nickel or nickel equivalent tolerable in our alloydecreases from 12% with a chromium content of 10%, to about 9% with achromium content of about 14%. We have found that unless the nickel ornickel equivalent content is kept below 12% for the smaller amounts ofchromium, that is, between about 8 to 10% chromium, and unless it iscarefully reduced to 9% as the chromium content is increased from 10 to14%, an excessive amount of stable austenite,

that is more than about 10%, is retained in the composition, and theunique properties of our alloy are not attainable.

The critical interrelationship of the chromium and nickel or thechromium plus nickel and copper content of our alloy may be bestunderstood with reference to the drawing in which the percent chromiumis plotted along the abscissa and the percent nickel equivalent isplotted along the ordinate. The area ABCDEF clearly brings out themanner in which the chromium, nickel and copper content in our alloy isbalanced at each level of chromium and nickel within the ranges of 8 to14% chromium and 5 to 12% nickel. In practicing the broader aspects ofour invention, chromium and nickel or chromium and nickel equivalentcontents are balanced so that when their contents expressed as percentare plotted respectively along the abscissa and ordinate of theaccompanying drawing, the point defined thereby will lie on or withinthe area 3 ABCDEF. Thus, for a given chromium content of 12%, theminimum nickel or nickel equivalent content must be about 6% and themaximum must not exceed about 10.5%. Similarly, if the nickel or nickelequivalent content of the alloy is desired to be about 9.5%, then thechromium content may range from about 8 /2 %1 to 13.3

Our age hardenable stainless martensitic alloy is especially suitablefor making parts having intricate shapes because of the ease with whichit may be fabricated in its annealed condition from which, by arelatively simple heat treatment, it is converted to an age hardenedmartensite having a unique combination of strength, toughness,durability and hardness. Better tensile strength, notch tensile strengthand ductility in our alloy are attained in accordance with our inventionby controlling the chromium and nickel or the chromium and nickelequivalent content of our alloy so that it falls within the morerestricted area AHDEF as shown in the accompanying drawing. Here, whilethe chromium content may range from 8 to 14%, the nickel content or thenickel equivalent content of our alloy ranges from 10 to 12% for 8%chromium, to to 9% when the chromium content is as high as 14%. In thearea AHDEF, an ultimate tensile strength of about 225,000 p.s.i. or moreis attainable. However, we prefer to utilize a minimum of 7% nickel ornickel equivalent for consistent attainment of high notch strength andductility.

To enhance the stainless properties of our alloy, we utilize a minimumof 10% chromium and at least 1% copper in our alloy so that it fallswithin the area BCDEK. The better strength and ductility together withenhanced stainless properties are thus attained in our alloy when it iscontrolled so as to fall within the area GHDEK.

Fabricators of parts having intricate shapes, which in use are subjectedto stress, require that the alloy utilized be sufliciently strong andductile so that the part is not likely to fail because of high stressconcentrations that occur at sharp angles or fine cracks. In accordancewith our invention, when the chromium and nickel or the chromium andnickel equivalent content of our alloy are maintained so as to fallwithin the more restricted area AH'NLF', not only is the ultimatetensile strength of our alloy equal to at least 225,000 p.s.i. or more,but with a stress concentration factor (K of 10 in the notch, the notchtensile strength of any given analysis in that area is greater than theultimate tensile strength. In other words, the ratio of the notchtensile strength and the ultimate tensile strength is greater than oneand the part fails in the smooth part of the test specimen beforefailing at the notch.

As shown in the drawing, in the area AHNLF', chromium ranges from about8% to 13.5% while nickel or the nickel equivalent range from about 7.8%to 12%, to provide high strength in our alloy combined with exceptionalnotch ductility. To enhance corrosion resistance we use at least about10% chromium and at least about 1% copper as was pointed outhereinabove. Thus, our alloys falling in the area GH'NLM arecharacterized by a unique combination of stainless and mechanicalproperties. Furthermore, to provide maximum corrosion resistance weprefer to limit carbon to 0.03% and for best results to .01% or less.

In addition to the careful balance of the elements chro mium, nickel andcopper in our alloy, it is also necessary to carefuHy control theamounts of titanium, aluminum, columbium and boron. In our alloy, atleast .6% titanium is required to impart hardness and strength, whileabove about 2%, titanium tends to cause embrittlement of the alloy. Forbest results, we preferably utilize titanium in an amount ranging from.75 to 1.6%. However, when the larger amounts of chromium, that is inthe neighborhood of 14%, are present in our alloy, best results areattained with from about .75 to 1.2% titanium.

Columbium works to enhance notch ductility and toughness and for thispurpose we utilize columbium in an amount ranging from about .25 toabout 1%. Below about .25%, the desired effect is not attained. Aboveabout 1%, columbium does not appear to have any beneficial effect andbecause it is a strong austenite stabilizer, columbium may upset themartensitic balance of the alloy when present in amounts in excess ofabout 1%. Preferably, columbium in an amount ranging from about .35% to.65% is used.

Aluminum is a desirable addition in our alloy in amounts ranging up to.5% and preferably when used is not present in an amount in excess of35%. When present, aluminum works together with the titanium to ensuremaximum strength with high notch ductility in our alloy. When bothaluminum and titanium are present in our alloy, their combined contentshould not exceed about 1.5%. Because titanium is a powerful austenitestabilizer, aluminum in the stated amount of up to about .5 isadvantageously utilized as a substitute for a like amount of thetitanium to provide ultimate tensile strength in the neighborhood of290,000 p.s.i. However, when maximum transverse ductility is requiredrather than maximum ultimate strength, titanium without aluminum ispreferably utilized. Boron is also useful in enhancing the transverseductility of parts formed from our alloy and has a beneficial effectupon the hot workability of our alloy in amounts ranging up to about.1%. Preferably, our alloy includes from about .002% to .005 boron.

Carbon is not an essential constituent of our alloy and preferably islimited to no more than .03%. Carbon is more or less tolerable in ouralloy, depending upon its intended use. Most notably affected by thepresence of larger amounts of carbon are the transverse properties(e.g., transverse ductility and toughness) of parts such as bars havinga substantial cross section. Larger amounts of carbon may also tend todetract from the corrosion resistance of our alloy. Thus, carbon ispreferably limited to .03 maximum and best results are achieved whencarbon is limited to no more than .0l%. On the other hand, when thebetter transverse properties of the alloy are not required, as forexample, when the alloy is provided in the form of sheet material,larger amounts of carbon can be present in our alloy so long as it isnot present in suificient quantity to upset the martensitic balance ofthe alloy or adversely affect the desired strength. Because carbon tendsto tie up titanium and columbium which are believed to be involved inthe hardening, strengthening and/or toughening mechanism of our alloy,no more than about .2% carbon should be present in our alloy unless theelements titanium and columbium are present in the amounts designated bythe upper portions of their respective ranges.

While the best properties of our alloy are brought out when suchtechniques as melting under vacuum are utilized, good ingots well suitedfor many less demanding uses are obtained when the alloy is made usingconventional air melting techniques such as air induction or aremelting. Best results are attained when the alloy is made by firstcasting an ingot using air induction or are melting practices or vacuuminduction melting, forming the ingot thus obtained into an electrode andremeltin-g it under vacuum following conventional vacuum consumableelectrode arc melting techniques.

The alloy is solution treated (that is, austenitized) by heating to1400-1700 F. for from one quarter of an hour to two hours followed byair cooling. In some instances it may be desirable to use a two stepsolution treatment in which event the alloy is first heated to from1700-1900 F. followed by air cooling and reheating to 1400 to 1600 F.and again air cooled. In its solution treated condition, our compositionis soft and may readily be worked as desired to form a wide variety ofintricate shapes. Cooling below room temperature is not required. Theparts which have been formed from the alloy in its soft condition arestrengthened, hardened and toughened by aging for a suitable length oftime at a temperature of about 800 to 1100 F. followed by cooling inair. The

optimum duration of the aging treatment may be readily determined foreach analysis. In practice, aging for from two to sixteen hours hasproven satisfactory although shorter aging treatments, fifteen minutesor even less, may prove advantageous.

Age hardenable martensitic stainless steel alloys illustrative of ourinvention are set forth in Table I. Unless otherwise indicated, each ofthese alloys as well as those set forth following Table I, was hammerforged to bars from ingots cast under vacuum. The bars were solutiontreated or annealed at about 1500" F. for about one-half hour, formedinto test specimens which were then aged for eight hours at about 950 F.

TABLE 1 Ex. C Cr N1 Ti Al Cb Cu B In the alloys of the foregoingexamples and in those set forth hereinafter unless otherwise noted,manganese and silicon each were present in amounts less than .1% andphosphorus and sulfur were below .01%. For boron, the amount shown wasthe amount added.

TABLE IL-MECHANICAL PROPERTIES Percent Percent Ultimate Notch Ex.Hardness, Elonga- Reduction Tensile Tensile No Rockwell C tion of AreaStrength, Strength,

p.s.1. p.s.1.

Solution treated at 1500 F. and aged at 900 F.

The hardness of the alloys of Examples 1-9 was also measured onspecimens in the annealed or solution treated condition and all werefound to be between Rockwell C 28 and Rockwell C 32. Smooth ultimatetensile strength specimens were readily formed having a gauge length ofone inch, a .252 inch gauge diameter and one-half inch diameter threadsat each end. The notch tensile strength specimens having a stressconcentration factor (K of 10 were formed 4 /2 inches long having/2-inch diameter threads at each end, a diameter of .357 inch notched to.252 inch diameter with a root radius of .001 inch and a 60 notch angle.

TABLE I11 Ex. C Cr Ni Ti Al Cb Cu B N o.

As in the case of the examples in Table 1, Examples 10-13 were vacuummelted and contained less than .1% manganese, less than .1% silicon,less than .01% phosphorus and less than .0l% sulfur, the balance ofExamples 1-l3 was essentially iron in each instance.

TAB LE IV.-MEGHANICAL PROPE RTIES Percent Percent Ultimate Notch Ex.Hardness, Elonga- Reduction Tensile Tensile No. Rockwell 0 tion of AreaStrength, Strength,

p.s.i. p.s.1.

In marked contrast to the alloys of the present inven tion set forth inTable 1, Examples 10, 11 and 13 exhibit extremely poor notch tensilestrength, while Example 12 is characterized by extremely low ultimatetensile strength. Furthermore, Examples 10, 11 and 13 exhibit theeffects of poor ductility and Example 12 although exceedingly ductileillustrates the adverse effect of excessive retained austenite upon thestrength of the alloy.

As was described in connection with Examples l-9, the alloys of Examples14 and 15 were prepared having analyses as set forth in Table V. Testspecimens Were made from the alloys of Examples 14 and 15 also as wasdescribed in connection with Examples l-9 except that the parts wereaged for four hours instead of eight hours at about 950 F. Themechanical properties thereof set forth in Table VI were obtained fromtests carried out on the specimens as was described in connection withthe examples of Table I.

TABLE V Ex. No.

C Cr Ti Al Cb Cu B Al was less than 03%.

As in the case of the examples in Table 1, Examples 14 and 15 werevacuum melted. While the silicon content of each was .04%, Examples 14and 15 contained less than .0l% manganese, less than .0l% phosphorus,less than .0l% sulfur and the balance was essentially iron in eachinstance.

dicated a hardness in their aged condition of Rockwell C 44 but thatalloy is capable of being aged to a higher hardness level by using asomewhat lower aging temperature without substantially affecting theother properties of the alloy. When a specimen of Example 15 Wasprepared and heat treated as was previously described except that agingwas carried out at 900 F., it was found to have a hardness of 46Rockwell C.

It is apparent from Examples 14 and 15 that it is the criticalinterrelationship between the chromium and nickel or nickel plusone-half the copper content in our alloy which determines itsoutstanding properties of ultimate and notch tensile strength andductility, and as was noted hereinabove, these properties are not undulyaffected by the larger amounts of carbon so long as the martensiticbalance of the alloy is not upset. Specimens of Examples 14 and 15 intheir heat treated condition showed no more retained austenite, lessthan about 3%, than would be consistent with their position in the graphof the drawing, slightly above the line AGH, determined by theirchromium content and their nickel plus one-half their copper content.

The outstanding properties of our alloy are readily obtained when it isprepared in commercial quantities so as to contain a maximum of .01%carbon, about 10.75%

7 to 11.5% chromium, about 8.5% to 9% nickel, about 1% to 1.25%titanium, about .35% to .45% columbium, about 1.2% to 1.8% copper, about002% to 004% boron, a maximum of .1% manganese,'a maximum of .1%silicon, a maximum of .01% each of phosphorus, sulfur and nitrogen withthe remainder consisting essentially of iron. For example, a 5,000 poundingot was prepared by first melting a heat in a vacuum induction furnaceand casting it into a 14-inch round ingot which was then remelted in avacuum consumable electrode furnace to a 20-inch round ingot having thefollowing analysis in percent by weight:

1 Balance except for incidental impurities.

The thus formed ingot was readily hot worked to a 9 inch square billet.A section was cut from the billet and solution treated at 1500" F. forone-half hour and then water quenched. As solution treated, the alloyhad a hardness of Rockwell C 30. Test specimens were prepared from thesection as was previously described in connection with Examples 1-9 andwere then age hardened at 950 F. for eight hours followed by air coolingto a hardness of Rockwell C 48. The specimens thus prepared had a roomtemperature ultimate tensile strength of 235,000 p.s.i. and a notchtensile strength of 290,000 p.s.i. The .2% yield strength was 223,000p.s.i. with an elongation of 11% and with 49% reduction in area.

The 9 inch square billet was further hot worked to 4 inches square and,after surface preparation, was hot rolled to bars about .813 inch round.Following solution treatment at about 1500 F. for one-half hour andwater quenching, test specimens were prepared from these rounds as waspreviously described in connection with Examples 1-9. The test specimenswere aged by heating at about 900 F. for about 4 hours followed by aircooling to a hardness of Rockwell C 51. The specimens thus prepared hada room temperature ultimate tensile strength of 257,000 p.s.i. and anotch tensile strength of 301,000 p.s.i. The .2% yield strength was245,000 p.s.i. with an elongation of 12% and with 47% reduction in area.

The following test is illustrative of the corrosion resistance of ouralloy. A specimen having the analysis of Example 1 was prepared in theshape of a tensile specimen and was immersed in a boiling aqueoussolution of 6% sodium chloride and 1.5% sodium dichromate (Na Cr O withthe specimen loaded at about 75% of its .2% yield strength, that is, atabout 190,000 p.s.i. The thus tested specimen failed after 97 hours. Twosuch specimens of essentially identical analysis as Example 1 weresubjected to the same test at a load of about 184,000 p.s.i. with onefailing at 265 hours and the other at 318 hours. Specimens ofessentially the identical analysis as Example 1 were also subjected tosalt spray at 95 F. for 14 days without undergoing any noticeablediscoloration.

The high strength and ductility of our alloy coupled with its goodstainless properties and the readiness with which it may be aged to ahardness of at least Rockwell C 45 make it eminently well suited for awide variety of uses.

The terms and expressions which havexbeen employed are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recog-'nized that various modifications are possible within"the scope of theinvention claimed.

We claim:

1. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 10% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of chromium and a nickel equivalent selected from the groupconsisting of nickel and nickel plus up to about 7%- copper in amountssubstantially in accordance with the area ABCDEF in the accompanyingdrawing with cop per replacing no more than about 4% nickel and in whichthe nickel equivalent content is calculated by adding one-half thepercent copper to the percent nickel, up to about .2% carbon, about .6%to 2% titanium, up to about .5% aluminum,about .25% to 1% columbium, upto about .1% boron, up to about 2% manganese, up to about 1% silicon,and the remainder consisting essentially of iron except for incidentalimpurities.

2. The stainless steel alloy as set forth in claim 1 in which saidnickel equivalent is at least about 7%.

3. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 10% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of chromium and a nickel equivalent selected from the groupconsisting of nickel and nickel plus from, about 1 to 7% copper inamounts substantially in accordance with the area ABCDEF in theaccompanying drawing with copper replacing no more than about 4% nickeland in which the nickel equivalent content is calculated by addingone-half the percent copper to the percent nickel, up to about .2%carbon, about .6% to 2% titanium, up to about .5% aluminum, about .25%to 1% columbium, up to about .1% boron, up to about 2% manganese, up toabout 1% silicon, and the remainder consisting essentially of ironexcept for incidental impurities.

4. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 10% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of chromium and a nickel equivalent selected from the groupconsisting of nickel and nickel plus from about 1 to about 7% copper inamounts substantially in accordance with the area ABCDEF in theaccompanying drawing with copper replacing no more than about 4% nickeland in which the nickel equivalent content is calculated by addingone-half the percent copper to the percent nickel, up to about .03%carbon, about .75 to 1.6% titanium with no more than about 1.2% titaniumwhen chromium is present in an amount of about 14%, up to about 35%aluminum, about 35% to .65 columbium, about 002% to .005 boron, up toabout 2% manganese, up to about 1% silicon, and the remainder consistingessentially of iron except for incidental impurities.-

5. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 1% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of chromium and nickel equivalent selected from the groupconsisting of nickel and nickel plus from about 1 to about 7% copper inamounts substantially in accordance with the area AHDEF in theaccompanying drawing with copper replacing no more than about 4% nickeland in which the nickel equivalent content is calculated by addingone-half the precent copper to the percent nickel, up to about .2%carbon, about .6% to 2% titanium, up to about .5% aluminum, about .25%to 1% columbium, up to about .1% boron, up to about .2%

manganese, up to about l% silicon, and the remainder consistingessentially of iron except for incidental impurities.

6. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 1% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of chromium and a nickel equivalent selected from the groupconsisting of nickel and nickel plus from about 1 to about 7% copper inamounts substantially in accordance with the area AHDEF in theaccompanying drawing with copper replacing no more than about 4% nickeland in which the nickel equivalent content is calculated by addingone-half the percent copper to the percent nickel, up to about .03%carbon, about .75 to 1.6% titanium with no more than about 1.2% titaniumwhen chromium is present in an amount of about 14%, up to about .35%aluminum, about 35% to .65% columbium, about .002% to .005% boron, up toabout 2% manganese, up to about 1% silicon, and the remainder consistingessentially of iron except for incidental impurities.

7. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structure withno more than about 1% retained austenite, said alloy having high tensilestrength and high notch tensile strength and consisting essentially ofchromium and a nickel equivalent selected from the group consisting ofnickel and nickel plus from about 1 to about 7% copper in amountssubstantially in accordance with the area AH'NLF' in the accompanyingdrawing with copper replacing no more than about 4% nickel and in whichthe nickel equivalent content is calculated by adding onehalf thepercent copper to the percent nickel, up to about .2% carbon, about .6%to 2% titanium, up to about .5% aluminum, about to 1% columbium, up toabout .1% boron, up to about 2% manganese, up to about 1% silicon, andthe remainder consisting essentially of iron except for incidentalimpurities.

8. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 1% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of chromium and a nickel equivalent selected from the groupconsisting of nickel and nickel plus from about 1 to about 7% copper inamounts substantially in accordance with the area AHNLF in theaccompanying drawing wtih copper replacing no more than about 4% nickeland in which the nickel equivalent content is calculated by addingone-half the percent copper to the percent nickel, up to about .03%carbon, about .75 to 1.6% titanium with no more than about 1.2% titaniumwhen chromium is present in an amount of about 14%, up to about 35%aluminum, about .35 to .65% columbium, about .002% to .005% boron, up toabout 2% manganese, up to about 1% silicon, and the remainder consistingessentially of iron except for incidental impurities.

9. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 1% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of chromium and a nickel equivalent selected from the groupconsisting of nickel and nickel plus from about 1 to about 7% copper inamounts substantially in accordance with the area GHNLM in theaccompanying drawing with copper replacing no more than about 4% nickeland in which the nickel equivalent content is calculated by addingone-half the percent copper to the percent nickel, up to about .03%carbon, about .6% t0 2% titanium, up to about .5 aluminum, about .25% to1% columbium, up to about .1% boron, up to about 2% manganese, up toabout 1% silicon, and the remainder consisting essentially of ironexcept for incidental impurities.

10. A stainless steel alloy capable of being age hardened to a hardnessof at least 45 Rockwell C with an essentially martensitic structurehaving no more than about 1% retained austenite, said alloy having hightensile strength and high notch tensile strength and consistingessentially of no more than about .01% carbon, about 10.75% to 11.5%chromium, about 8.5% to 9% nickel, about 1% to 1.25% titanium, about .35to .45 columbium, about 1.2% to 1.8% copper, about .002% to 004% boron,no more than about .1% manganese, no more than about .1% silicon, andthe remainder consisting essentially of iron except for incidentalimpurities.

References Cited UNITED STATES PATENTS 2,381,416 8/1945 Wyche et al148-38 XR 2,447,897 8/ 1948 Clark -125 2,482,096 9/1949 Clark 75-1252,528,497 11/1950 Clark 75-125 2,747,989 5/1956 Kirkby 75-128 2,850,3809/1958 Clark 75-125 2,999,039 9/1961 Lula et a1. 148-37 3,210,22410/1965 Argo 148-142 3,258,370 6/1966 Floreen et al 75-128 XR 3,288,61111/1966 Lula et a1. 75-128 3,347,663 10/1967 Bieber 75-128 XR 3,357,86812/1967 Tanczyn 148-37 XR L. DEWAYNE RUTLEDGE, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.

